An apparatus for adaptive echo cancellation of the kind mentioned above has been previously described in Swedish patent application 8106444-6. As described in the Swedish application, echo cancellation of, for example, an incoming analogue signal is effected by correction signals from an adaptive balance filter, the correction signals being added to the incoming signal and the parameters of the balance filter being continuously or in stepwise fashion updated from a correction unit.
A known method of functionally controlling digital equipment is to loop circuit data flow from a transmitter to a receiver and to carry out a bit error measurement. In this way the transmitter, receiver and four-wire to two-wire junction of the echo cancellation apparatus can be functionally controlled.